The kinetic parameters K, and V,,, for urea uptake by Melosira italica were determined at 160 peinsteins m -2 S -1 and in the dark. The transport systems showed an affinity for the substrate and a storing capacity in the dark (K, = 65.07 pM; V,,, = 2.18 nmoles lo5 cells-' h -') greater than under 160 pE m -2 s -' (K, = 1 11.2 p M ; V,,, = 1.11 nmoles 10' cells -' h -'). Similarly, a reduction in consumption rate of urea under increasing photon flux densities was observed. The use of an inhibitor (potassium cyanide) indicated that the uptake process requires metabolic energy. That urea transport is more important in darkness, may constitute a survival strategy in which this compound is utilized by cells mainly during heterotrophic growth.
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